1. Field of the Invention
The present invention relates to vapor-deposited diamond and a method of producing the same, and more particularly, it relates to a technique for providing a large diamond single crystal which can be used for a semiconductor material, an electronic component, an to optical component or the like.
2. Description of the Background Art
Diamond, which has a number of excellent properties such as high hardness, high thermal conductivity, high light transmittance and a wide band gap, is widely used as a material for tools optical components, a semiconductor devices and electronic components, and its it may become even more important in the future.
Single crystals of natural diamond include a so called type IIa diamond single crystal which transmits ultraviolet light of up to 230 nm and also called type Ia diamond single crystal which hardly transmits ultraviolet light. In either case, it is extremely difficult to obtain a natural single crystal at least 10 mm in diameter. Although single-crystalline diamond close to 20 mm in diameter rarely occurs naturally, such diamond is too expensive for an industrial use. Natural type IIa diamond has a large amount of crystal defects and distortion. Such natural IIa diamond is regarded as unsuitable for use as a substrate for a semiconductor device, since it has an angular half-width of at least 500 seconds in an X-ray rocking curve and a half-width of at least 2 cm.sup.-1 in a spectrum of Raman scattered light observed at around 1332 cm.sup.-1. On the other hand, type Ia diamond cannot be used as an optical material for ultraviolet light, since it transmits no ultraviolet light having a wavelength less than or equal to 300 nm.
While natural diamond has been applied to industrial use in the past, artificially synthesized diamond is mainly applied to such industrial uses at present. At present, single crystal diamonds are industrially synthesized under a pressure of at least tens of 1000 atmospheres for stabilizing the diamonds. A superhigh pressure vessel for generating such a pressure is so expensive that it is difficult to increase the content volume of the vessel, and diamond cannot be supplied at a low cost. In such a high-pressure method, therefore, synthesis of a large single crystal is limited. Further, diamond which is produced by the high-pressure method is easily converted to a crystal called a type Ib diamond crystal, which contains nitrogen as an impurity. The type Ib diamond is relatively large, but it transmits absolutely no light of less than or equal to 400 nm in wavelength. Thus, it has been impossible to artificially synthesize a diamond single crystal at least 10 mm in diameter, which transmits ultraviolet light of around 250 nm.
On the other hand, vapor deposition can be used as a method of synthesizing diamond, which has been established with the high-pressure method. It is possible to artificially produce diamond which has a relatively large area of several to 10 cm, while such diamond is generally in the form of a polycrystalline film. When a diamond film is to be formed by vapor deposition, a substrate has generally previously been prepared by being scratched with diamond grains. It is understood that the effect of promoting the growth of a diamond film by scratching is attained due to fine particles of diamond that are left on the substrate and serve as seed crystals for growing the diamond (see e.g. S. Iijima, Y. Aikawa and K. Baba, Appl. Phys. Lett. 57 (1990), 2646). The diamond particles left on the substrate after scratching are directed in various orientations, and hence the diamond grown from the particles, which serve as seed crystals, forms a polycrystalline film. However, it is necessary to use single-crystalline diamond for ultraprecise tools optical components semiconductor devices, which require a smooth surface.
To this end, a method of epitaxially growing single-crystalline diamond by vapor deposition has been studied. In general, epitaxial growth is classified into homoepitaxial growth of growing a target material on the same type of materials a substrate, and heteroepitaxial growth of growing a target material on different types of substrates. A single-crystalline substrate of a relatively large area can easily be obtained for heteroepitaxial growth. At present, however, heteroepitaxial growth of diamond tends to cause a defect or distortion in the crystal, and it is thus unsuitable as a method of obtaining a large-area diamond single crystal to be is applied to an optical component or a semiconductor substrate. Therefore, it is still important to study homoepitaxial growth in order to produce a large-area diamond single crystal.
In relation to such homoepitaxial growth of diamond, various devices have been made in order to obtain large single crystals. Geis et al. have reported a method of arranging diamond particles several 10 to 100 .mu.m in size, which may be used as abrasive grains on a selectively etched Si substrate for growing diamond having a strong specific crystal orientation on this substrate (M. W. Geis, H. I. Smith, A. Argoitia, J. Angus, G. H. M. Ma, J. T. Glass, J. Butler, C. J. Robinson and R. Pryor, Appl. Phys. Lett., Vol. 58 (1991), p. 2485).
Japanese Patent Laying-Open No. 3-75298 (1991) and U. S. Pat. No. 5,127,983 corresponding thereto disclose a method of arranging a plurality of single-crystalline diamond plates several mm square for growing diamond thereon from a vapor phase. According to this method, it is possible to obtain a large diamond crystal which can be regarded as a single crystal as to light transmittance and the like, although grain boundaries having extremely small differing inclinations may be present in boundaries between the substrates. In particular, it is disclosed that a practicable large diamond single crystal can be obtained by precisely controlling crystal orientations of and spaces between the plurality of diamond plates.
In order to obtain a diamond single crystal of at least 15 mm in size according to U.S. Pat. No. 5,127,983, it is important to maintain homoepitaxial growth up to a prescribed thickness. While the method of U.S. Pat. No. 5,127,983 is remarkably effective for epitaxially growing a large diamond single crystal, the formation of particles (hereinafter referred to as abnormally grown particles) losing the epitaxial relation between the particles and the substrates is not completely suppressed. In this known method, it is possible that abnormally grown particles are generated in boundaries between the substrates. As shown in FIG. 1, an abnormally grown particle 2 is generated in a boundary region between two substrates 1 and 1' with a probability higher than those in other regions. Therefore, it is important to reduce the probability of forming abnormally grown particles in consideration of deposition of larger single-crystalline diamond.